Filed of the Invention
The invention relates to a method for operating a once-through steam generator having a combustion chamber. The containing wall of which is formed from vertically disposed evaporator tubes welded to one another in a gas-tight manner and a flow medium flowing through the evaporator tubes. It relates, further, to a once-through steam generator for carrying out the method. A steam generator of this type is known from the article "Verdampferkonzepte fur Benson-Dampferzeuger"["Evaporator Concepts for Benson Steam Generators"] by J.Franke, W. Kohler and E. Wittchow, published in VGB Kraftwerkstechnik 73 [VGB Power Station Technology 731(1993), issue 4, pages 352-360. In the case of a once-through steam generator, the heating of the evaporator tubes forming the combustion chamber or the gas flue leads to the flow medium evaporating in the evaporator tubes in a single pass. In contrast, a natural-circulation or forced-circulation steam generator has only partial evaporation of the circulated water/steam mixture. In this case, the evaporator tubes of the once-through steam generator may be disposed vertically or spirally and therefore at an inclination.
In contrast to a natural-circulation steam generator, a once-through steam generator is not subject to any pressure limitation. Therefore, it is possible to have fresh-steam pressures well above the critical pressure of water (P.sub.crit =221 bar), where there is only a slight density difference between the liquid-like and steam-like medium. A high fresh-steam pressure is conducive to high thermal efficiency and therefore low CO.sub.2 emissions of a fossil-heated power station. A once-through steam generator, the gas flue of which is composed of vertically disposed evaporator tubes, can be produced more cost-effectively than a spiral version.
Furthermore, once-through steam generators with vertical tubing have lower steam-side pressure losses, as compared with those having evaporator tubes that are inclined or are disposed so as to ascend spirally.
A once-through steam generator having a combustion chamber, the containing wall of which is formed from vertically disposed evaporator tubes welded to one another in a gas-tight manner, is known from Published, Non-Prosecuted German Patent Application DE 43 33 404 Al.
A particular problem is to configure the gas-flue or the combustion-chamber wall of the once-through steam generator to allow for the tube-wall or material temperatures occurring there. In the sub-critical pressure range up to about 200 bar, the temperature of the combustion-chamber wall is determined essentially by the value of the saturation temperature of the water when it is possible to ensure wetting of the heating surface in the evaporation region. This is achieved, for example, by using evaporator tubes which have a surface structure on their inside. Internally ribbed evaporator tubes come under consideration particularly for this purpose, the use of these in once-through steam generators being known, for example, from Published European Patent Application No. 0 503 116. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
In the pressure range of about 200 to 221 bar, the heat transmission from the tube inner wall to the flow medium decreases sharply, with the result that the mass flow density of the flow medium has to be selected correspondingly high, in order to ensure sufficient cooling of the evaporator tubes. For this purpose, the mass flow density must be selected higher in the evaporator tubes of once-through steam generators, which are operated at pressures of about 200 bar and above, than in the case of once-through steam generators which are operated at pressures of below 200 bar. However, a mass flow density increased in this way also results in a higher pressure loss in the evaporator tubes due to friction. As a consequence of this higher pressure loss due to friction, the advantageous property of vertical tubing, namely that, in the case of multiple heating of an individual evaporator tube, its throughput also rises, is lost, particularly in the case of small tube inside diameters. However, since steam pressures of above 200 bar are necessary for high thermal efficiency and low CO.sub.2 emissions of a power station, it is necessary, in the pressure range too, to ensure good heat transmission from the tube inner wall to the flow medium. Once-through steam generators having a vertically tubed combustion-chamber wall are therefore normally operated at relatively high mass flow densities. In this respect, the publication "Thermal Engineering", I. E. Semenovker, Vol.41, No. 8, 1994, pages 655 to 661, uniformly specifies a mass flow density at 100% load of about 2000 kg/m.sup.2 s both for gas-fired and for coal-fired once-through steam generators.